1. Field of the Invention:
The present invention relates to a circuit for counting the number of tracks of an optical disk crossed over by an optical head in an optical disk apparatus.
2. Prior Art:
FIG. 4 illustrates a conventional circuit for counting the number of tracks crossed over by the optical head, in which a tracking error signal 1 representing a deviation of the optical head from the track is converted into, for example, a binary signal by a binarization circuit 2, which signal is counted by a counter 3. The counted value is stored in a register 5 for each sampling cycle indicated by a sampling signal 4 and, at the same time, the value of the counter 3 is reset. Thus, the count of the number of the crossed tracks for each sampling cycle is stored in the register 5.
Next, a seeking operation (which means movement of the head from one track to another) by using the counted number of crossed tracks is described. During the seeking operation, control of the head speed is conducted. That is, in order to carry out an optimum seeking operation, a current head speed V is controlled so as to follow a target speed V.sub.p. In addition, since the count of the crossed track number for a single sampling cycle is the number of tracks by which the head advances within a predetermined period of time, this value can be translated into speed and, from the count of the number of crossed tracks for each sampling cycle, the head speed V for each sampling cycle can be evaluated. Here, the head is controlled so that V.sub.p -V may become small (If the head speed V is smaller than the target speed V.sub.p, then a positive force corresponding to this difference is applied to the head. On the other hand, if V is larger than V.sub.p, then a negative force corresponding to this difference is applied to the head.), and, when V becomes sufficiently small and the number of tracks crossed over by the head, which is evaluated by summing the counts of crossed tracks for each sampling cycle, reaches the target number of tracks to be sought, the seek operation is completed.
However, the above-described conventional circuit for counting the number of crossed tracks has a drawback that, when the optical head crosses a mirror section, preformatting section and a defective section (for example, about 2700 byte preformatting section is crossed over in the access of 6000 tracks), the tracking error signal is lost, resulting in the count error of the number of crossed tracks. Once this count error occurs, the number of crossed tracks is counted less than the actual value, and the head speed would be recognized smaller than the actual head speed and, as the result, a force stronger than necessary is applied to the head, which can cause the head to race. In addition, the count error of the number of crossed tracks causes inaccurate counting of the number of crossed tracks, which makes the seeking error or a positional difference between the target track and the head position at the end of seeking greater.